Well, the 7970M is faster than a 675M, yet it draws a similar amount of power. Same holds true in CF/SLI. I would choose the Radeon cards in this case.
Well, the 7970m is 2 tiers higher on the gpu hierarchy. So it isn't really even arguable which one is stronger. The 7970m will definitely be stronger. You might want to look up battery life reviews for the laptops you are looking at because they might use a lot of power.
BUMMER!!! This is for my new Alienware M18x-R2. Its a replacement from my old one which has only GTX660M. So they gave me an upgrade from 660M to 675M SLI. I called them and they can not change the order anymore because they already sent it to the Factory which the AW will be assembled. It's ok as long as i can play High on the specs that is fine. But Ultra would have been better : )